Non-invasive methods of monitoring brain activity have long been sought. More recently, systems that allow a patient to perform activities while monitoring brain activity, known as functional monitoring have been sought which allow neurological and psychological study of the patient. Many types of diagnoses and studies are made possible including studies that involve brain development in children, brain activity in mentally deficient patients, patients that have experienced brain damage or concussions, conditions such as migraines, and in the elderly, conditions such as Alzheimer's disease.
The technique of near infrared spectroscopy and functional near infrared spectroscopy have been shown to be useful in obtaining images of subcutaneous matter including fluids and solid matter. Near infrared spectroscopy is accomplished by transmitting light through the skin from a near infrared light source and into the subcutaneous tissue where it is scattered and absorbed. The scattered light is detected by a detector, usually coupled to an optical fiber placed a short distance away from the transmission source. Near infrared images of subcutaneous matter are generally wavelength dependent due to the light absorption characteristics of water, of oxygenated hemoglobin (HbO) and of deoxygenated hemoglobin (Hb). Observation of three dimensional spatial variations of hemoglobin concentration is possible using multiple wavelengths of light. Most often, lasers or LED sources in the ranges of 690 nm to 830 nm are employed.
Near-infrared spectroscopy has shown to be a promising tool in breast imaging for early detection of breast cancer, peripheral vasculature for diagnostics relating to peripheral blood flow and arterial health which are common issues in diabetics, and in related research applied to animals.
Depth of the imaging field is in the range of centimeters: about 3 to 4 cm for brain imaging, about 10-12 cm for breast imaging, and about 6 to 8 cm for the peripheral body such as the arms and legs.
Commercial systems exist in the art. FIG. 1A shows a typical prior art system. Patient 1 wears cap 2. The cap includes optical fibers held in close proximity to the patient's skull. The optical fibers include bundle of transmit fibers 3 and bundle of receive fibers 4. Bundle of transmit fibers 3 are connected to a set of lasers on a set of transmitter cards 5. Bundle of receive fibers 4 are connected to a corresponding set of optical detectors on a set of receiver cards 6. The set of transmitter cards and the set of receiver cards are controlled by computer 8 having memory 7. The computer operates to collect data from the receiver cards and process it using image processor 10 to determine various imaging data. For example, hemoglobin concentration may be determined as a function of position on the skull and at various depths within the brain. Display 9 is used to display the processed images and hemoglobin concentration maps.
In FIG. 1B, a prior art transmitter card is shown. The transmitter card includes a set of lasers 21 operating at the two wavelengths, 690 nm and 830 nm. Typically, there are four lasers of each wavelength on a card, where each laser includes fiber pigtail 25 to an external fiber optic connector 22. Fibers 27 from bundle of transmit fibers 3 are separated. Each fiber includes a connector which is mated to connector 22.
In FIG. 1C, a prior art receiver card is shown. The receiver card includes a set of detectors 18. Avalanche photodiode detectors (APD) are typically included for detecting low light levels. In most cases, four photodetectors are included on each card. Each photodetector includes fiber pigtail 20 to external fiber optic connector 23. Fibers 28 from the bundle of receive fibers are separated. Each fiber terminated with a fiber connector which is mated to external fiber optic connector 23. The transmitter cards and the receiver cards are connected to and interact with the computer.
FIG. 2 shows a prior art diagram depicting a detail of cap 2 in operation. A patient's head including skin 39 covering subcutaneous fluid 40 is shown. Membrane 38 supports and positions input fiber 30, and output fibers 31-36. Near infrared light, when injected through input fiber 30 is scattered by the subcutaneous fluid in all directions. However, certain scattering paths allow for the light to propagate into the output fibers. An example is central blood flow artifact 48. Light path 44 captures light from input fiber 30 to output fiber 31. Light path 42 captures light from input fiber 30 to output fiber 32. Light path 43 captures light from input fiber 30 to output fiber 33. Light path 44 captures light from input fiber 30 to output fiber 34. Light path 45 captures light from input fiber 30 to output fiber 35. Light path 46 captures light from input fiber 30 to output fiber 36. The “banana” shaped light paths are statistical in nature describing paths that photons take while propagating in the subcutaneous fluid from the light source towards the detector. In so doing, some of the photons are absorbed. The absorption is exponentially related to the path length and to artifacts. In particular, photons propagating in the region of central blood flow artifact 48 will experience a higher rate of absorption as determined by the extinction coefficient of the material comprising central blood flow artifact 48. Thus, light detected in output fibers 33 and 35, in particular, will be less than their counterparts, output fibers 34 and 36, respectively. Also, note that output fibers 35 and 36 probe deeper into the subcutaneous fluid than output fibers 31 and 32, for example.
In the prior art, Gratton et al. in U.S. Pat. No. 5,497,769 discloses the quantitative determination of various materials in highly scattering media such as living tissue in an external, photometric manner by the use of a plurality of light sources positioned at differing distances from a sensor. The light from said sources is amplitude modulated in accordance with conventional frequency domain fluorometry techniques where the gain of the sensor is modulated at a frequency different from the frequency of the light modulation. The sensor heads carry eight light sources and the light passing through the living tissue may be transmitted to a photomultiplier detector by an optical fiber.
Barbour et al., in U.S. Pat. No. 7,778,693 discloses a time series of optical tomography data obtained for a target tissue site in a human using a near infrared optical wavelength to observe properties of the vasculature of the human. A target placed in an imaging head is exposed to optical energy from combined sources. A source demultiplexer is controlled by a computer to direct the optical energy source fibers sequentially. The imaging head contains a plurality of source fibers and detector fibers for transmitting and receiving light energy, respectively. The optical energy entering the target at one location is scattered and may emerge at any location around the target where it is collected by detector fibers. The imaging process is repeated so as to deliver optical energy sequentially, a measurement being obtained for detected emerging optical energy at each detector for each emitting source fiber. Barbour et al. discloses a system with 32 detection channels.
In U.S. Pat. No. 7,983,740 to Culver et al., an imaging system for diffuse optical tomography is disclosed including a dense grid of light emitting diodes as sources wherein each light emitting diode has individual, isolated power to reduce crosstalk and each detector channel has a dedicated avalanche photo diode. The separation of signals is carried out through decoding frequency encoding.
These prior art systems suffer from a number of deficiencies. A first deficiency is in the size and portability of the system. As researchers and physicians have gained experience with these systems, they have seen the need for a larger numbers of detectors and sources in order to increase resolution. Current systems have as many as 128 detectors or transmitters. Such a system would require a fairly large rack of equipment and substantial space to operate. Furthermore, the sheer numbers and cost of the electronics become prohibitively large. Second, prior art systems operate at frame rates of about 2.5 Hz or less, at low resolutions and small coverage areas. For larger coverage areas the frame rates deteriorate to less than 1 Hz.
A compact near infrared hyperspectral imaging system is disclosed by Livingston et al. in U.S. Patent Application Publication No. 2008/0306337 that discloses an apparatus and method of the use of a hyperspectral surgical laproscope comprising a liquid crystal tunable filter mounted on the laproscope, positioned to collect back-reflected light from a target, and focal plane array also mounted on the laproscope to image light reflected from the target.
A digital light processing hyperspectral imaging apparatus is disclosed by Zuzak et al. in U.S. Patent Application Publication No. 2010/0056928, the system including an illumination source adapted to output a light beam, the light beam illuminating a target, a dispersing element arranged in the optical path and adapted to separate the light beam into a plurality of wavelengths, a digital micromirror array adapted to tune the plurality of wavelengths into a spectrum, an optical device having a detector and adapted to collect the spectrum reflected from the target and arranged in the optical path and a processor operatively connected to and adapted to control at least one of these components and further adapted to output a hyperspectral image of the target.
However, these prior art systems only provide images of the spectral response of the tissue area and depth illuminated as a whole without regard for localized photonic excitation and scattering. As a result, lateral and depth spatial resolutions as well as image contrast of these systems remain limited.